1. Field of the Invention
The present invention relates to a video signal processing device for converting an analog video signal into a digital signal having an appropriate dynamic range, and a television receiving device including such a video signal processing device.
2. Description of the Related Art
Video signals for color television are commonly broadcast as composite signals including luminance (Y) and color difference (C) information. For digitizing such a video signal, a video signal processing device for appropriately controlling the dynamic range of an analog/digital converter is used. Such a technique is disclosed in, for example, Japanese Patent Laid-Open Publication Nos. Hei 8-125122 and 2001-356831.
Referring to FIG. 5, a typical conventional video signal processing device 100 includes a channel switching circuit 10, an input level detecting circuit 12, a programmable amplifier 14, and a digital/analog converter (A/D converter) 16. The channel switching circuit 10 includes a multiplexer operating based on a channel selection signal provided from an external source so as to select and provide one of a plurality of input video signals. The input level detecting circuit 12 receives the video signal selected by the channel switching circuit 10, detects an amplitude of the video signal, and outputs a gain control signal for adjusting an amplification ratio of the programmable amplifier 14 based on the detected amplitude. The programmable amplifier 14 receives the gain control signal from the input level detecting circuit 12, and amplifies and outputs the video signal selected by the channel switching circuit 10 at the amplification ratio in accordance with the gain control signal. The A/D converter 16 receives the output from the programmable amplifier 14, converts the analog signal into the digital signal, and outputs the converted signal.
By way of example, when the gain control signal provided from the input level detecting circuit 12 is 3 dB, the video signal is amplified twice in the programmable amplifier 14 before input to the A/D converter 16, as illustrated in FIG. 6. The A/D converter 16 outputs a digitized video signal digitized with a dynamic range in accordance with the difference between a voltage Vt applied to a maximum reference voltage terminal VRT and a voltage Vb applied to a minimum reference voltage terminal VRB. For example, when the voltages Vt and Vb are 2 V and 0 V, respectively, the potential difference is 2 V. The amplitude of the video signal is then amplified to the level of approximately 2V by the programmable amplifier 14, thereby fully utilizing the dynamic range of the A/D converter 16 to digitize the signal.
Thus, a wide dynamic range of the digitized video signal is secured by adjusting the amplitude of the video signal in line with the dynamic range of the A/D converter 16 using the programmable amplifier 14 in conventional video signal processing devices. However, a circuit for forming the programmable amplifier 14 is complicated, resulting in an increase in the size of the chip area required for the video signal processing device. In addition, reliable realization of favorable properties remains a difficult and complicated process.